One of the most vexing problems in the use of surfactant flooding for enhanced oil recovery is the frequent substantial loss of surfactant due to adsorption on the formation matrix and precipitation by polyvalent cations such as calcium and magnesium. Surfactant adsorption on the formation matrix significantly decreases surfactant flood efficiency, and because it is necessary to inject a greater quantity of surfactant, increases the cost of any surfactant flood.
Additionally, most surfactants are satisfactory for surfactant flooding only if the calcium and magnesium concentrations of the formation water fall below about 500 ppm. Petroleum sulfonates, the most popular type of surfactants, precipitate where divalent ion concentrations exceed about 500 ppm. Such precipitation renders the sulfonates inoperative for recovering oil and in some instances, causes formation plugging.
Many subterranean petroleum-containing formations are known to exist which contain polyvalent ions such as magnesium and calcium in concentrations far in excess of 500 ppm. The most common of these reservoirs are limestone formations which may have polyvalent ion concentrations from 200 to 20,000 ppm in the original connate water. Similar polyvalent ion concentrations can also be found in sandstone reservoirs containing kaolinite and bentonite clays, which also provide additional problems in adsorption of surfactant.
Where high divalent ion concentrations exist, most petroleum sulfonates cannot be used because the high surfactant losses due to precipitation and adsorption on the matrix render use uneconomical. In such an environment, the flood water will lack the surfactant necessary to substantially decrease the interfacial tension between water and petroleum. Furthermore, precipitated petroleum sulfonate often plugs small flow channels in subterranean hydrocarbon formations. Such plugging from precipitated surfactants decreases formation porosity and injectivity, causing substantial decreases in oil displacement efficiency.
Nonionic surfactants, such as polyethoxylated alkyl phenols, polyethoxylated aliphatic alcohols, carboxylic esters, carboxylic amides and polyoxyethylene fatty acid amides have a somewhat higher tolerance of polyvalent ions than do the more commonly utilized anionic surfactants which generally have water soluble sulfonate, sulfate, phosphate or carboxylate groups. However, while it is technically feasible to employ a nonionic surfactant solution to decrease interfacial tension between the injected aqueous displacing medium and the petroleum contained in hydrocarbon formations, these surfactants are frequently not economically feasible for several reasons. First, nonionic surfactants are also subject to adsorption on the formation matrix, which can drastically decrease the amount of surfactant available for lowering interfacial tension. Second, nonionic surfactants are not as effective on a per unit basis as are the more commonly used anionic surfactants. Third, nonionic surfactants have a higher cost per unit weight than do anionic surfactants. Furthermore, nonionic surfactants are ineffective at formation temperatures above their cloud points.
Various chemicals have been employed as sacrificial agents to pretreat formations to decrease the adsorption of subsequently injected surfactants or to tie up polyvalent cations and prevent them from precipitating surfactants from the flood medium. Some examples include the use of aqueous solutions of pyridine as disclosed in U.S. Pat. No. 3,414,054, the use of sodium carbonate and inorganic polyphosphates as disclosed in U.S. Pat. No. 3,469,630, the use of metal phosphates as disclosed in U.S. Pat. No. 3,688,844 and the use of modified lignosulfonates as described in U.S. Pat. Nos. 4,133,385; 4,142,582 and 4,172,497.
U.S. Pat. No. 3,692,113 discloses the injection of a polyalkylene glycol of high molecular weight, probably greater than one million to further increase the viscosity of the flooding medium to prevent channel formation within the reservoir. The polyalkylene glycol is preferably injected with alkaline earth materials. U.S. Pat. No. 3,882,939 also discloses the injection of a polyalkylene glycol to increase viscosity of the flooding solution, but with a molecular weight of from 10,000 to 10,000,000, preferably from 1 to 6 million. The polyalkylene glycol was injected in conjunction with a biopolymer in '939 to achieve better mobility control.
U.S. Pat. Nos. 3,946,811 and 4,005,749 disclose the use of polyalkylene glycols as sacrificial agents with surfactant and polymer floods to reduce surfactant loss to divalent ion precipitation and adsorption within the formation. In U.S. Pat. No. 3,946,811 it is stated that the polyethylene glycol should have a molecular weight of from 10,000 to 100,000,000, most preferably 100,000 to 600,000. A molecular weight greater than 1,200 is stated to be required for the practice of U.S. Pat. No. 4,005,749. The reference examples consider and reject the use of a polyalkylene glycol with a lower molecular weight.